[leetcode] Count Primes

Count Primes

Description:

Count the number of prime numbers less than a non-negative number, n

click to show more hints.

References:

How Many Primes Are There?

Sieve of Eratosthenes

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

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class Solution
{
public:
  int countPrimes(int n)
  {
    vector<int> a(n);
    for(int i=0; i<n; i++)
      a[i] = i;

    for(int i=1; i * 2<= n; i++)
    {
      if(i == 1) 
      {
        a[i] = 0;
        continue;
      }

      for(int j = i + i; j < n; j += i)
      {
        if(a[j] != 0)
          a[j] = 0;
      }
    }

    int count = 0;
    for(int i=0; i<n; i++)
    {
      cout << a[i] << " ";
      if(a[i] != 0)
      {
        count++;
      }
    }
    cout << endl;

    return count;
  }
};